Generic pressure-measuring cells have a ceramic measuring membrane and a ceramic counter body, said measuring membrane is connected pressure-tight with the counter body along a circumferential joint, especially having an active brazing solder, wherein a pressure chamber is formed between the measuring membrane and the counter body, wherein the equilibrium position of the measuring membrane arises from the difference between a pressure prevailing in the pressure chamber and a pressure acting on the outer side of the measuring membrane facing away from the pressure chamber. Generic pressure-measuring cells also comprise a capacitive transducer for converting the pressure-dependent deformation of the measuring membrane into an electrical signal.
In particular, aluminum oxide ceramics, which are suitable for the production of pressure-measuring cells due to their elastic properties and their resistance to media, are used as the material for the counter body and the measuring membrane. The ceramic components mentioned are joined, in particular using an active brazing solder that preferably contains Zr—Ni—Ti. The production of such active brazing solder is disclosed, for example, in the European published patent application EP 0 490 807 A2. According to the method described in the publication, rings that must be positioned between the measuring membrane and the counter body to solder them together can especially be produced from the active brazing solder material.
For example, niobium, tantalum or silicon carbide are known as electrode materials for electrodes of the capacitive transducer, wherein the surface of the electrodes is to be optionally protected by a glass layer, as described in the publication EP 0 544 934 A1. Similarly, electrodes are known which comprise metallic particles in a glass matrix and are described, for example in the published German patent application DE 10 2007 026 243 A1.
The electrode materials mentioned are suitable for joining aluminum oxide ceramics using an active brazing solder in a high-temperature vacuum soldering process.
However, the temperature range in which an active brazing solder forms a high-quality pressure-tight connection with a ceramic material is comparatively narrow. At too low temperatures, the solder is not sufficiently reactive on the one hand and it is too viscous to spread evenly on a surface area to be wetted on the other.
However, at too high temperatures, there is a risk that the solder has such a low viscosity that it enters areas that are not meant to be wetted by it.
However, in the production of larger batches of measuring cells, it is inevitable that a temperature distribution that exploits the available temperature range is given in an oven. Nevertheless, provision of a solder stop that limits the spread of the active solder is known to obtain useful results.
A practiced method for limiting the radially inwardly flowing active brazing solder is oxidation of the surface of a membrane electrode, which comprises tantalum, and should be in galvanic contact with the active brazing solder. At relatively low soldering temperatures, entry of the active brazing solder into the pressure chamber can be prevented with an acceptable yield. However, if the soldering temperature is increased, the solder stop no longer acts reliably, and the solder flows over the edge of the tantalum electrodes into the pressure chamber. Apart from the narrow temperature range in which the pressure-measuring cells described are to be soldered, there is a further disadvantage that the electrodes now have at least two layers with different thermal coefficients of expansion, namely a metallic tantalum layer and a tantalum oxide layer. This may result in temperature-dependent mechanical stresses on the surface of the measuring membrane which cause a temperature hysteresis.